This is a Wordpress function which uses a timestamp in seconds and microseconds, cast to 22 decimal places. See this code reference for the detail.
Empirically it seems that casting a timestamp to 22 digits adds pseudo-random junk to the least significant part:
x=$(date +%s.%N) # %N is nanoseconds, 9 digits. Microseconds would be
# 6 digits but the principle is similar enough
printf "%s -- %.22F\n" "$x" "$x"
1515706488.768975900 -- 1515706488.7689759000204503536224
As an aside, the human-readable date "11/01/2018" as written in your question is ambiguous. In some parts of the world it reads as you (and I) would consider "01/11/2018". Furthermore the time part (and actually, potentially the date too) will vary depending on your time zone. For that reason it would be better to describe 1515663000 as being equivalent to "11 Jan 2018 09:30:00 UTC".